This invention relates to a drive unit for a motorcycle that includes an internal combustion engine having at least two cylinders arranged in a V-configuration, a clutch and a transmission, i.e. a gear box.
The V-type arrangement of cylinders in motorcycle engines has a number of advantages which have led to the widespread use of such engine layouts. In the instance of two-cylinder engines the small overall width obtained with this arrangement will permit large banking angles of the motorcycle even with low-lying engines.
On the other hand, such a cylinder arrangement will increase the overall length of the drive unit, which will have its disadvantages, especially in the instance of large-displacement engines with a V-angle of 90xc2x0. It has been found moreover that in the instance of relatively short rear wheel swing arms, the quality of ride and handling will undergo a considerable change when the rear wheel oscillates, thus affecting driving stability.
It is an object of the present invention to avoid these disadvantages and to further develop a drive unit of the above described type such that the available space will be fully utilized whilst an advantageous rear axle geometry is achieved.
According to the invention the transmission is disposed in the space between the cylinders. Since this will permit the transmission to be accommodated within the space taken up by the engine at any rate, a particularly compact configuration of the drive unit will be obtained.
Preferably, a gear is provided on the crankshaft of the engine which meshes with a gear connected to a main gearshaft via the clutch. The primary drive is thus effected by a pair of gears. This solution is facilitated by the close vicinity of main gearshaft and crankshaft due to the layout proposed by the invention.
In a preferred solution the engine is provided with camshafts which are driven by gears in mesh with the gear on the main gearshaft. This gear whose outer contour extends into the region of the cylinder heads, is particularly well suited for driving the camshaft. In general, one intermediate shaft carrying two gears will suffice to bridge the distance to the camshaft axis on the one hand and provide the required transmission ratio on the other hand.
Special preference is given to a variant of the invention which provides that both a transmission output shaft carrying a sprocket for the rear wheel drive, and the bearing for the rear wheel swing arm be positioned in the space between the cylinders. In addition to the compact arrangement of components this variant will offer the advantage of a comparatively long rear wheel swing arm. As a consequence, the geometry of the chassis will change only slightly when the rear wheel oscillates and the quality of ride and handling will be upheld.
Preferably, the axle of the sprocket is situated higher than the axle of the rear wheel such that the rear wheel swing arm will drop at an angle of about 10xc2x0 without load. Under normal load the rear wheel swing arm will thus drop slightly, whilst it will be level or slightly raised in the compressed state.
A further advantage is obtained if engine crankshaft, main gearshaft and transmission output shaft are positioned in a common plane. In particular, this will permit engine block and transmission housing to be split in the common plane. From the point of view of strength, such horizontal splitting is desirable in high-performace engines as no vertical splitting will be necessary in this case. The common plane will subtend an approximately equal angle with both cylinder axes in most instances, deviations being possible depending on the configuration of the transmission.
In an alternative version of the invention engine block and transmission housing are provided as an integral part. This variant will offer greater liberty in designing the transmission, as the countershaft need not be positioned in the same plane as crankshaft and main gearshaft.
A compact design is also achieved by providing the clutch outside of a cover plane of the engine. In this manner the diameter of the clutch will not be constrained by the cylinder arrangement.
In arranging the engine components more space may be saved by providing that the cylinders have a V-angle of 75xc2x0 to 105xc2x0 approximately. Angles of about 90xc2x0 will offer sufficient space to accommodate the transmission as well as a satisfactory balancing of masses.
The present invention further relates to a motorcycle with a drive unit of the above type, wherein one cylinder is essentially horizontal and another cylinder is essentially vertical, the horizontal cylinder preferably being oriented against the direction of travel. In this manner the wheelbase may be kept particularly short.
Oil drainage from the horizontal cylinder is enhanced by providing that the axis of the horizontal cylinder have an inclination angle xcex1 of about 5xc2x0 to 15xc2x0 relative to the horizontal plane, rising towards the cylinder head.
Oil consumption and friction losses can be reduced by providing an oil scraper configured as a baffle plate in the engine crankcase, which plate will largely cover the opening into the horizontal cylinder. In this way the oil flow from the crankshaft drive into the cylinder will be minimized.
Ride and handling of the vehicle may be improved by positioning the axle of the sprocket higher than the axle of the rear wheel such that the rear wheel swing arm will drop at an angle of about 10xc2x0 against the horizontal in the unloaded state.